The present invention relates to air flow dampers used to control or affect the flow of air through a duct, into or out of a duct, or between two volumes. For instance, low wall return dampers are used for any return air system to balance the airflow across spaces. A damper installed in a return system can be adjusted from the room side to distribute airflow across the space for proper air recirculation. Similarly, dampers can be used on a duct output such as in a forced air HVAC system to help control the amount of air flow through a particular location.
Such dampers have one or more movable plates which control the characteristic dimensions of one or more orifices through which the air flows. In many dampers, the movable plate(s) rotate about an axis which is transverse to the air flow direction, with the rotation causing the projected amount of surface area of the movable plate restricting air flow (i.e., looking parallel to the air flow direction) to change. In other dampers referred to as slide plate dampers, the plate(s) always extend perpendicular or at least transverse to the air flow direction, and the movement direction of the plate(s) is perpendicular/transverse to the air flow direction. The movement of the slide plate could be linear, or in some instances is rotational about an axis parallel to the air flow direction. The present invention particularly applies to slide plate dampers and similar arrangements, such as disclosed in U.S. Pat. Nos. 5,218,998 and 7,597,617, both incorporated in full by reference, wherein the plates extend generally transverse to the air flow direction through the plate, and wherein the orientation of the plates relative to the air flow direction doesn't change.
For instance, the damper of U.S. Pat. No. 5,218,998 uses two plates with numerous openings in each plate, with flat sides of the plates adjacent or against each other. One plate is generally fixed in place while the other one moves. The relative sliding movement of the plates causes the percentage of the open area in one plate which overlaps with the open area in the other plate to change, i.e, sliding changes the sizes of the orifices through the plate combination. Larger orifices provide less resistance to airflow, smaller orifices provide more resistance to airflow. The combined ribbing of the two plates creates back pressure, which can be used to increase air pressure drop and to direct airflow to other side of the space to create uniform airflow and avoid dead spots.
In such prior art air dampers, the opening size is commonly smaller than the web between openings, so two plates can be aligned to create no orifices and thus be used to fully shut off flow. The general thinking is that the flow resistance is a function (not necessarily a linear function, but still a function) of total orifice area.